Why not just try in incorporate one of these on the cockpit somewhere (stowing it when not in use) and have a good workout as well as charging up the batteries?
That's another RE idea that pops up from time to time: human-powered electrical generation. It's not too bad in some ways, but its primary value is illustrating (in ways we can comprehend) just how freakin' difficult it is to make meaningful amounts of electrical power. A fairly fit athlete -- and most cruisers are not all that
-- can produce 1/4 to 1/3 horsepower during vigorous exercise: bicycling, rowing, x-country skiing. Maybe half a horsepower in short bursts, but figure a quarter to a third ongoing.
One horsepower is 746 watts. Pedaling or rowing like the devil, with an efficient direct-drive PM alternator, will produce ~200W. That's real electricity for sure, but it also means you'd need to hold that pace for five hours
to generate as much power (1 kWh) as 200W of solar panels on a medium-sunny day. Your refrigeration may use that much juice in a warm climate.
It's good exercise, mind you; and in the RE world, some Watts is better than no Watts, as long as the means of generating them don't excessively ruin other aspects of the boat, like weight distribution, windage, stowage room, or cruising budget.
Speaking or ruining the budget ... a Watt&Sea tow generator will produce that same 1 kWh in about four hours sailing at 6kts. It also only works while under way. And a high-output aux generator or engine alternator will take just about an hour to bank the same. No surprise that, faced with depleted batteries and set of bicycle pedals, most sailors would reach for the ignition key.